Proteases are involved in many facets of lung biology, especially those processes characterized by cellular emigration, proliferation, and tissue reorganization: inflammation, fibrosis, and emphysema. Regulated expression of one enzyme system, urokinase (uPA) and its cognate receptor (uPAR), is strongly linked to this pathobiology and this is partly due to the prominent role of uPA/uPAR in regulating pericellular proteolysis. However it has recently been discovered that uPAR has a dual function, that of an adhesion receptor for vitronectin, a multifunctional matrix protein deposited at sites of injury and within the stroma of several tumors. On this basis of this duality it is proposed that uPA, uPAR, vitronectin, and an inhibitor of uPA (plasminogen activator inhibitor type1, PAI-1), operate as a system to regulate the adhesive and proteolytic phenotype of uPAR-bearing cells. In monocytes/macrophages, uPAR is the major vitronectin receptor and its function in adhesion is tightly coupled to the activation state of the integrin receptor Mac-1 (CD11b/CD18). Experiments are directed at defining the molecular pathways which connect uPAR function with that of Mac-1. Toward that end both chimeric Mac-1 and uPAR receptors will be co-expressed in eukaryotic cells to explore the structural determinants important to their interaction. Experiments are also proposed to define in vitro the importance of uPAR- vitronectin interactions to cellular processes linked to uPAR expression: migration and proteolysis. The thesis that uPAR and PAI-1 may cooperate in attachment/detachment of cells to promote migration will be specifically addressed. This thesis is based on accumulating clinical information linking PAI-1 expression with metastasis and poor survival of lung cancer patients. The potential role of uPAR in phagocytosis of vitronectin- opsonized particles and bacteria will also be defined. Finally, targeted mice without a functional uPA gene will be employed to test whether concepts and results generated by in vitro experiments can be extended to in vivo settings. Both acute and chronic models of lung injury will be employed to test whether uPA and uPAR directly contribute to cellular emigration within the lung, to innate immunity against infection, and to the tissue remodeling that develops in the course of injury and the inflammatory response. These experiments should determine whether future attempts to manipulate the uPA/uPAR system in patients susceptible to inflammatory, fibrotic, and metastatic lung diseases have a rational basis.